As is well known in the art, the combustion of hydrocarbon-based fuels in an internal combustion engine produces as a byproduct several undesirable oxides of nitrogen (NOx emissions). The release of such NOx emissions is tightly regulated by governmental authorities in many parts of the world. Exhaust gas recirculation (“EGR”), in which exhaust gases are recirculated to the engine's intake manifold in order to undergo further combustion, is a proven method for reducing NOx emissions. Unfortunately, EGR is difficult to implement on turbocharged engines, such as turbocharged diesel engines, for example. This is because turbocharged engines often have a mean exhaust manifold pressure below the mean intake manifold pressure near peak torque output operating point (“torque peak”), such that the exhaust gases will not automatically flow to the intake manifold if a connection is made between the intake and exhaust manifolds.
Until recently, engine designers could compensate for a lack of EGR at torque peak by providing extra EGR at high engine speeds, resulting in an acceptable average level of NOx emissions. But U.S. governmental regulations taking effect in 2002 require substantial NOx reductions at all engine speeds and loads involved in typical operation. In order to satisfy these regulations it will be necessary to utilize EGR at almost all engine operating points. A particular problem is how to obtain sufficient EGR at or near torque peak without compromising performance elsewhere.
Thus, there is a need for an EGR system that is capable of providing EGR at all speeds and loads, including torque peak, without harming engine performance at other conditions. The present invention is directed towards meeting this need.